My Samsung S4 experienced the same thing after an automatic software update a few weeks ago and I took it to a guy in a local shopping centre to fix. He tried to reverse the update but that wouldn't work so it had to be "flashed" (no idea, don't ask me) it cost £20 to fix and if works fine now.
There's a warning but, you may lose some data such as photos, videos or conversations on whatsapp as I did so you may want to download important stuff and save it elsewhere before handing over the 'phone.